I checked show hidden files but did not find a Chrome.css file, just the thunderbird file under etc folder with a syspref.js file.

That URL you included is from 2009. And. In a few quick searches for config info for 'userChrome.css' etc. ... all the results are also circa 2008-2009.

While some users with 'old' Firefox / Thunderbird profiles that have been move from machine to machine over time may still have 'userChrome.css', 'userChrome-example.css', 'userContent-example.css' in their '../cache' directory, I don't think that current Firefox / Thunderbird use the userChrome.css config file(s) any longer?.